1. Field of the Invention
The present invention relates to a method and apparatus to compensate for drift in magnetic field strength in superconducting magnets, particularly drift caused by thermal variation of magnetic properties.
2. Description of the Prior Art
FIG. 1 illustrates a schematic radial cross-section through a typical superconducting magnet structure used in an MRI system, such as may be improved by the present invention. A cylindrical magnet 10, typically having superconducting coils mounted on a former or other mechanical support structure, is positioned within a cryostat, has a cryogen vessel 12 that contains a quantity of liquid cryogen 15, for example helium, which holds the superconducting magnet at a temperature below its transition temperature. The magnet is essentially rotationally symmetrical about axis A-A. The term “axial” is used in the present document to indicate a direction parallel to axis A-A, while the term “radial” means a direction perpendicular to axis A-A, in a plane which passes through the axis A-A.
The cryogen vessel 12 is itself cylindrical, having an outer cylindrical wall 12a, an inner cylindrical bore tube 12b, and substantially planar annular end caps (not visible in FIG. 1). An outer vessel 14 surrounds the cryogen vessel. It is commonly referred to as Outer Vacuum Chamber (OVC), and will be referred to herein as OVC. The OVC 14 is also cylindrical, having an outer cylindrical wall 14a, an inner cylindrical bore tube 14b, and substantially planar annular end caps (not visible in FIG. 1). A hard vacuum is provided in the volume between the OVC 14 and the cryogen vessel 12, providing effective thermal insulation. A thermal radiation shield. 16 is placed in the evacuated volume. This is typically not a fully closed vessel, but is essentially cylindrical, having an outer cylindrical wall 16a, an inner cylindrical bore tube 16b, and substantially planar annular end caps (not visible in FIG. 1). The thermal radiation shield 16 serves to intercept radiated heat from the OVC 14 before it reaches the cryogen vessel 12. The thermal radiation shield 16 is cooled, for example by an active cryogenic refrigerator 17, or by escaping cryogen vapour.
In alternative arrangements, the magnet is not housed within a cryogen vessel, but is cooled in some other way: either by a low cryogen inventory arrangement such as a cooling loop, or a ‘dry’ arrangement in which a cryogenic refrigerator is thermally linked to the magnet. In ‘dry’ configurations, heat loads on the magnet are not directly cooled by liquid cryogens but, instead, are removed via a thermal link connected to a cooling pipe or refrigerator. Such heat-loads can result, for instance, from current ramping or gradient coil operation. The OVC is however still present.
The OVC bore tube 14b must be mechanically strong and vacuum tight, to withstand vacuum loading both radially and axially. Conventionally, it is made of stainless steel. The cryogen vessel bore tube 12b, if any, must be strong and capable of withstanding the pressure of cryogen gas within the cryogen vessel. Typically, this is also of stainless steel. The bore tube 16b of the thermal radiation shield 16 must be impervious to infra-red radiation. It is preferably lightweight and a good conductor of heat. It is typically made of aluminium.
In order to provide an imaging capability, a set of gradient coils 20 (not visible in FIG. 1) are provided within a gradient coil assembly 22 mounted within the OVC bore 14b. A gradient coil assembly 22 usually comprises a hollow cylindrical, resin-impregnated block, containing coils which generate orthogonal oscillating magnetic field gradients in three dimensions. A patient bore 25, located within the gradient coil assembly 22, is an open volume into which a patient is placed for imaging. A shimming arrangement is typically provided within the gradient coil assembly 22, where passive iron shims are placed in selected locations to improve the homogeneity of the background field B0 in the imaging region. Typically, the shims are placed in trays (not shown) which are, in turn, placed within shim slots 24.
During an imaging procedure, the gradient coils 20 generate rapidly oscillating magnetic fields with very fast rise-times of typically just a few milliseconds. Stray fields from the gradient coils generate eddy currents in metal parts of the cryostat, in particular in metal bore tubes 14b, 16b, 12b of OVC, thermal shield and cryogen vessel. These eddy currents cause ohmic heating of the OVC bore tube. Mechanical vibration of the gradient coil assembly causes vibration of the OVC bore tube 14b. As this vibration takes place within the magnetic field of the superconducting magnet 10, further currents are induced in the material of the OVC bore tube, causing further heating. The gradient coils themselves are made of resistive wire, typically copper, and heat significantly in use.
These factors combine to produce an appreciable heating of the OVC bore tube. Cryogen vessel 12 and thermal radiation shield 16 are cooled by liquid cryogen 15, where used, and refrigerator 17. They do not heat appreciably when the magnet is in use.
The superconducting magnet 10 operates in persistent mode and generates a constant magnetic field, which may be referred to as the “background field” B0.
An imaging region 30 is provided, typically near the radial and axial centre of the patient bore 25. Great care is taken to ensure that the background field B0 is homogeneous and constant throughout the volume of the imaging region. This is typically designed and achieved to within a few parts per million.
However, some temporal drift in both the homogeneity and field strength of the background field B0 is observed with time, when the MRI system is in use. This has been attributed to changes of magnetic properties of the material of the OVC bore tube 14b with varying temperature. The heating of the OVC and shims will cause higher order drifts leading to reduced homogeneity.
When the MRI system is in use, the OVC bore tube 14b is heated by conduction and radiation from the gradient coil assembly, and is heated by eddy currents caused by mechanical oscillation of the OVC bore tube, itself caused by interaction with time varying magnetic fields generated by the gradient coil assembly, as explained above.